Functionality of porous starch obtained by amylase or amyloglucosidase treatments
Introduction
Starch is widely used in food and industrial applications as a thickener, colloidal stabilizer, gelling agent, bulking agent and water retention agent (Singh, Kaur, & McCarthy, 2007). In general, native starches produce weak-bodied, cohesive, rubbery pastes when heated and undesirable gels when the pastes are cooled (Abbas, Khalil Sahar, & Meor Hussin, 2010). In order to meet its intended function, physical, chemical or enzymatic modifications are applied to achieve the functional properties not found in native starches (Jayakody and Hoover, 2002, Lacerda et al., 2008). The use of enzymatic modification has a number of advantages comprising fewer by-products, more specific hydrolysis products and high yield, besides better control of the process and end products with particular properties. There are many enzymes used to alter starch structure and to achieve desired functionality (Rosell & Collar, 2008). Enzymes hydrolyze (1 → 4) or (1 → 6) linkages between α-d-glucopyranosyl residues. The most common enzymes for starch modification include α-amylase, β-amylase, glucoamylase, pullulanase, and isoamylase. α-Amylase can hydrolyze the (1 → 4)-α-glucosidic bonds of starch in an endo-action. Hydrolysis occurs in a random fashion at any (1 → 4)-linkage within the starch chain to rapidly reduce the molecular size of starch and the viscosity of the starch solution during pasting. Amyloglucosidase is an exo-acting enzyme that catalyzes the hydrolysis of both α-d-(1 → 4) and α-d-(1 → 6)-linkages from the non-reducing ends of the starch chain. Numerous researchers have investigated enzymatic hydrolysis of starches from cereals, roots, tubers, and legumes in terms of enzyme adsorption, action pattern, extent of hydrolysis, degree of crystallinity and hydrolysis products (Colonna et al., 1992, Gallant et al., 1992, Hoover, 2001, Kimura and Robyt, 1996, Li et al., 2004). Taking into particular account that corn starch makes up more than 80% of the world market for starch, many researchers have been focused on the hydrolysis products released from the enzymatic reaction (Huang et al., 2010, Khatoon et al., 2009, Li and Ma, 2011, Miao et al., 2011). Therefore, kinetics mechanism of the enzymatic reaction and hydrolysis products have been studied by other researchers but the features of the modified starches have been the focus of a few studies. Simultaneous hydrolysis of waxy corn starch with amylase and amyloglucosidase have been reported for modifying the digestion of the starch (Miao et al., 2011), and reducing the digestibility of corn starch by partial α-amylase treatment (Han et al., 2006), or through the action of β-amylase or maltogenic α-amylase with transglucosidase (Ao et al., 2007). In addition, Lacerda et al. (2008) studied the thermal properties of corn starch treated with fungal α-amylase showing higher action on the amorphous region of the granule.
Lately, there is an increasing interest for the developing corn porous starch due to it has interesting properties for being used in the areas of food, medicine, chemical industry, cosmetics, agriculture and other fields. In fact, porous starch might be used in foods to ensure a steady release of spices, sweeteners, acid condiments, flavorings, or even to protect from light or oxygen highly oxidized compounds (Zhang et al., 2012). Porous starch is a modified starch that contains microsized pores on the surface and could be extended to the inner part of the granule. Previously, it has been reported its production by glucoamylase catalysis combined with ultrasonic treatment (Wu, Du, Ge, & Lv, 2011) and more recently Zhang et al. (2012) proposed the combination of α-amylase and glucoamylase optimizing the kinetic reaction for increasing the yield. Nevertheless, there is no information about the contribution of each enzyme to the starch changes. Because of modified starch is widely used in food formulations, it is of particular interest to determine biochemical features of starch and how they affect its functional properties. The aim of this research was to determine the independent effect of fungal α-amylase (AM) or amyloglucosidase (AMG) on corn starch at sub-gelatinization temperature, with special emphasis on biochemical features, thermal and structural analyses of treated starches.
Section snippets
Materials
Corn starch samples were generously supplied by Huici Leidan (Navarra, Spain). The enzymes used were of food grade. Fungal α-amylase (AM) (Fungamyl 2500 SG) and amyloglucosidase (AMG) (amyloglucosidase 1100 L) were provided by Novozymes (Bagsværd, Denmark). Chemical reagents from Sigma–Aldrich (Madrid, Spain) were of analytical grade.
Sample preparation
Preliminary assays were carried out for optimizing enzymatic reactions (starch quantity and pH), and pH 4.0 was selected for AMG reaction and pH 6.0 in the case of
Results and discussion
Enzymatic modification of the corn starch was carried out independently with α-amylase or amyloglucosidase under optimal pH conditions, 6.0 for α-amylase and 4.0 for amyloglucosidase. The action of each enzyme was compared with their specific control, which was submitted to the same treatment in the absence of enzymes. Results will reflect the effect of the pH and the enzymes on the starch features after being treated at sub-gelatinization temperature (50 °C). Zhang et al. (2012) found that 50 °C
Conclusion
Demands of modified starches are increasing in parallel to the rapid development of food industry. This study showed that enzymatic modification of corn starch by α-amylase or amyloglucosidase at sub-gelatinization temperatures led to porous starch granules that differed in both, the microstructure surface and the internal morphology. Results confirmed that the loss of granular structural order and changes in both amorphous and crystalline domains during sub-gelatinization temperatures can be
Acknowledgements
Authors acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness (Project AGL2011-23802), the European Regional Development Fund (FEDER) and Generalitat Valenciana (Project Prometeo 2012/064). A. Dura would like to thank predoctoral fellowship from Spanish Ministry of Economy and Competitiveness.
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